A common cavity resonator is the quarter wave transverse-electromagnetic (TEM) coaxial resonator (“TEM resonator”). In the TEM resonator the electric and magnetic fields lie in a transverse plane perpendicular to the conductors. The magnetic field is circular about the inner conductor. The electric field is axially symmetric about the inner conductor and extends from the inner conductor to the outer conductor. Current flows in the lengthwise direction along the surfaces of the conductors, in a direction perpendicular to both the electric and magnetic fields.
Two common characteristics or specifications used to determine/specify the performance of a TEM resonator are the length of the resonator and the Quality factor (“Q”). The length is generally specified as a quarter, or three quarter wavelength. This reflects the fact that the length of the resonator post is one-fourth or three-fourths of the length of the wavelength at the resonant frequency. The resonator post is formed by electrically shorting or connecting one end of the line, and leaving the other end open or electrically disconnected.
The quality factor Q of the resonator describes the sharpness of the system's response to input signals. A general definition of the quality factor Q that applies to acoustic, electrical, and mechanical systems, defines Q as equal to two times the product of the number π (pi) and the ratio of the maximum energy stored at resonance to the energy dissipated per cycle. In an electrical circuit, energy is stored in the electric or magnetic fields associated with reactive circuit components and electrical energy is lost (to heat) whenever current flows through a resistance.
TEM resonators can be used in various devices, including for example voltage controlled oscillators (VCO's), pagers, GPS (Global Positioning System) systems, TV/radio/cellular/PCS communications, magnetic-resonance imaging (MRI) systems and the like in frequency ranges from 10 MHz to 3 GHz. A variety of military systems utilize these frequencies and many must be frequency-agile. Furthermore, the increasing needs of homeland security and the more than 20 million radio users in the United States are requiring that more communications equipment be added to already over crowded sites. In addition, the private radio systems utilized by commercial and public safety industries continue to face capacity restraints.
There is an increasing need for high Q cavity resonators of reduced size so the space saved can be used for additional equipment. In addition, cavity resonators with higher performance and lower cost are also required in order to work in more complex communication applications, such as narrowband digital frequency hopping radios.